Steady-state and pre-steady state kinetic studies on the interaction of actin, myosin-subfragment-one (S-1) and ATP were carried out to better elucidate the mechanism of the actin activated myosin ATPase. We have already shown that it is not required for myosin to detach from actin during each cycle of ATP hydrolysis as was originally proposed by the Lymn-Taylor model. In the present study we provide evidence for the second major feature of our kinetic model---that there is a special rate limiting step, the transition from the refractory to the non-refractory state, which preceeds Pi release and which occurs with the myosin either attached to or detached from actin. By showing that the binding constant of M.ATP and M-ADP.Pi to actin is considerably weaker than the Kapp for ATPase activity under conditions where the magnitude of the initial Pi burst is not markedly decreased by actin, we can demonstrate that this special rate-limiting step must be present in the kinetic model. This rate-limiting step is of importance because it determines the rate at which cross-bridges enter the major force-producing state and thus it controls the velocity of muscle contraction.